Secure fax adapter method and apparatus

ABSTRACT

An secured fax adapter apparatus uses a relay circuit to selectively couple a G3 fax to a secured line via a secured telephone unit (STU). The G3 fax can alternatively be coupled to a public switched telephone network (PSTN) via a PSTN port. The secured fax adapter has an enhanced DAA having a controller, a fax communication circuit, a current limited off hook detection circuit, a relay circuit for selectively coupling the external device ports, and a ring generation circuit for producing a square wave for ring generation. In one embodiment, the current-limited off-hook detection circuit uses a symmetrical network of transistors coupled to diodes to limit the current drawn by a load. In another embodiment, the relay circuit has four relays for selectively coupling the G3 fax to the fax communication circuit, the STU to the PSTN, the G3 fax to the PSTN, or the STU to the fax communication circuit. The relay circuit has an additional relay for coupling the off-hook detection circuit to the fax communication circuit.

This application is a continuation of U.S. application Ser. No.08/513,015, Aug. 9, 1995 now U.S. Pat. No. 5,737,094.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to facsimile (fax) devices and more particularlyto fax adapter devices and methods of operation.

2. Description of the Related Art

Certain conventional fax machines transmit and receive non-secured datausing group 3 (G3) fax protocol over a Public Switched Telephone Network(PSTN). Certain other fax machines transmit and receive secured dataover a secured network using a secure protocol such as Secured TelephoneUnit (STU) protocol. In recent years, the cost of G3 fax machines hasdecreased and conventional G3 fax machines have proliferated. Theseconventional G3 fax machines however, cannot interface directly with anSTU. In certain instances, it is desirable to couple a conventional G3fax to a secured network via an STU. In these instances, secure faxadapters are used for coupling a conventional G3 fax to an STU.

Secure fax adapters conventionally use a Data Access Arrangement (DAA)to perform several Telephone Central Office (TCO) functions includingoff-hook detection and ring generation. Conventional DAA off-hookdetection circuits measure a voltage drop across a load to arrive at anon- or off-hook decision. Conventional off-hook detection circuits donot adequately current limit to protect defective or incompatibledevices coupled to the secure fax adapter. Additionally, someconventional off-hook detection circuits have a diode in the sensingsignal path which has the undesirable effect of introducing distortionto the sensing signal.

Ring generation is another TCO function typically performed by a securefax adapter DAA. The DAA of some conventional adapters use a transformerto generate a sinusoid for ring generation. Use of a transformer,however, is undesirable due to the added cost, weight and size.

Thus, there is a need for a low cost, light weight and compact securedfax adapter having an enhanced DAA with improved ring generation andoff-hook detection and providing increased flexibility in operatingconfiguration.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided an enhancedDAA having improved off-hook detection, ring generation andconfiguration flexibility. The enhanced DAA has a signal port forcoupling to a fax communication circuit, external device ports includinga STU port and a G3 fax port, an off-hook detection circuit, a ringdetection circuit, a ring generation circuit and a relay circuit.

In one embodiment, the relay circuit is configured to selectively couplethe G3 fax port to any of the STU port, the fax communication circuit orthe ring generation circuit, responsive to control signals received froma controller. In one embodiment, the controller is a CPU. In anotherembodiment, the DAA also has a PSTN port for coupling to a PSTN line. Inthis embodiment, the relay circuit additionally couples the STU port tothe fax communication circuit or to the PSTN port responsive to controlsignals received from the controller.

In another embodiment, the off-hook detection circuit has a currentlimiting circuit coupled to two sensing nodes and a detection circuitfor generating a logic signal corresponding to the signal sensed at thesensing nodes. The output of the detection circuit is a logic signalindicative of either on- or off-hook detection for coupling to thecontroller. The current limiting circuit has symmetrical currentlimiters, one for each sensing node. The current limiters each have atransistor having a base electrode coupled to a power supply through adiode to limit the current flowing through the transistor and to theload. The current limiting circuits are coupled to a detection circuitthat generates a logic signal indicating on- or off-hook detection. Thelogic signal is coupled to the controller.

In another embodiment, the ring generator generates a square wave ringsignal. The ring generator has a pair of switching circuits. A firstswitching circuit has a non-inverting conditioning circuit to receive asquare wave having a desired frequency from the controller. Thenon-inverting conditioning circuit output is coupled to a push-pullamplifier for generating an amplified square wave matching the frequencyof the square wave received from the controller. A second switchingcircuit has an inverting conditioning circuit to also receive the squarewave from the controller. The inverting conditioning circuit output iscoupled to a push-pull amplifier for generating an amplified, invertedsquare wave matching the frequency of the square wave received from thecontroller.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a secure fax adapter coupling a G3 fax toan STU in accordance with the present invention;

FIG. 2 is a block diagram of a secure fax adapter in accordance with thepresent invention;

FIG. 3A is a block diagram of an enhanced DAA in accordance with thepresent invention;

FIG. 3B is a schematic diagram of an enhanced DAA in accordance with thepresent invention;

FIG. 4 is a schematic diagram of a relay circuit in accordance with thepresent invention; and

FIG. 5 is a schematic diagram of a ring generation circuit in accordancewith the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

There is illustrated in FIG. 1, a block diagram of a secure fax adapter(SFXADP) 100 coupling a G3 fax 101 to a STU 102 in accordance with thepresent invention. SFXADP 100 has various user selectable routingconfigurations for adapting conventional G3 fax 101 to STU 102. SFXADP100 has external device ports 110-115 for coupling to conventionalexternal devices including a G3 fax 101, a STU 102, a personal computer103, a PSTN (a telephone subscriber line) port 105 and a power module104. Port G 110 couples SFXADP 100 to a conventional G3 fax 101.Alternatively, port G 110 of SFXADP 100 may be coupled to a modem orother device conforming to the CCITT Group 3 protocol of port G 110. Inthe preferred embodiment, port G 110 is a standard telephone SubscriberLine Interface Circuit (SLIC) consisting of two wires: a Data Tip (DT)and a Data Ring (DR) wire. Port S 112 couples SFXADP 100 to securedcommunication equipment such as an STU 102 and port H 111 couples SFXADP100 to a host such as a personal computer. Port P 114 is an auxiliaryphone port that can be internally linked to port L 113 to form a commonPSTN port. In the preferred embodiment, ports S 112 and H 111 areRS-232C ports. Power module 104 is coupled to the power input port 115of SFXADP 100. STU 102 is additionally coupled to a leased line 106.Alternatively, conventional STU 102 may be coupled to a PSTN, or coupledback to SFXADP 100 through port P 114. In the preferred embodiment,ports P 114 and port L 113 are RJ11 ports each having DT and DR wires.

There is illustrated in FIG. 2 a block diagram of SFXADP 100 inaccordance with the present invention. SFXADP 100 has a controller suchas a central processor unit (CPU) 220 for controlling fax communicationby receiving a timing signal from oscillator 204, and sending andreceiving control instructions to and from an erasable programmablememory (EPROM) device 216 and static random access memory (SRAM) 217. Ina preferred embodiment, CPU 220 is an Intel 80C188XL. A standarderasable programmable logic device (EPLD) 204, such as a model EP1810integrated circuit is also in communication with CPU (controller) 220 tocontrol serial communication controllers (SCC) 205, 206 and fax chip set(fax communication circuit) 201. Fax chip set 201 is a fax communicationcircuit which uses conventional fax protocol. In a preferred embodiment,fax chip set 201 is a model RC96V24AC fax chip set produced by Rockwell.

SCCs 205, 206 are serial communication controllers such as acommercially available model AM85C30 manufactured by Advanced MicroDevices of Sunnyvale, Calif. SCCs 205, 206 convert transmitted data froma parallel data format to a serial data format, and convert datareceived from either a secure fax network or a PSTN from serial data toparallel data. During transmission, fax chip set 201 transfers data todata bus 203. SCC 205 receives the data in parallel form from data bus203 and transfers data serially to transceiver 207. Transceiver 207thereafter transmits the serial data to port S 112 for coupling to anSTU 102. CPU (controller) 220 and SCC 205 set up automatic synchronousword, baud rate detection, and network information exchange for allstandard secure fax protocols which preferably are embedded or stored inEPROM 216. Similarly, SCC 206 receives parallel data from data bus 203and transfers data serially to transceiver 210. Transceiver 210thereafter transmits the serial data to port H 110 for coupling to aPC/AT 103. CPU (controller) 220 and SCCs 205, 206 set up automaticsynchronous word, baud rate detection, and network information exchangefor all standard PC/AT protocols which preferably are embedded or storedin EPROM 216.

CPU (controller) 220 sends and receives address information acrossinternal address bus 218. Internal address bus 218 is connected to databus 203 for receiving address information and is connected to latch 215for delivering address information along address bus 219 to either anerasable programmable read only memory (EPROM) 216 as a memory meanscontaining protocol software, or to a static random access memory (SRAM)217 as a memory means, depending on control instructions received fromthe CPU (controller) 270. In a preferred embodiment, EPROM 216 stores256K 8-bit words and SRAM 217 stores 512K 8- bit words. Data bus 203also couples data to both EPROM 216 and SRAM 217 as CPU (controller) 220controls memory read/write functions. Address bus 219 further conveyscontrol information from CPU (controller) 220 to EPLD 204, SCC 205, 206and fax chip set 201. CPU (controller) 220 directly conveys controlinstructions to EPLD 204, EPROM 216, SRAM 217, DAA 200, fax chip set201, SCC 205, 206, liquid crystal display (LCD) 212, and mode switch213.

There is illustrated in FIG. 3A a block diagram of an enhanced DAA 200in accordance with the present invention. DAA 200 has fax chip setinterface port 202, external device ports 221, 222, 223, an off-hookdetection circuit 301, a relay circuit 302, a ring detection circuit303, a ring generation circuit 305 and a PSTN line interface 304. Relaycircuit 302 is coupled to device ports 222, 223 and to device port 221via PSTN line interface 304. Relay circuit 302 is additionally coupledto ring generation circuit 303, to off-hook detection circuit 301 and tointerface ports 202.

There is illustrated in FIG. 3B a schematic diagram of a portion ofenhanced DAA 200 in accordance with the present invention. The schematicdiagram of FIG. 3B depicts a schematic diagram of off-hook detectioncircuit 301, relay circuit 302, ring detection circuit 303 and PSTN lineinterface 304.

FIG. 4 illustrates a schematic diagram of relay circuit 302 of DAA 200.Relay circuit 302 includes relays 401-405 and split transformer 407.Split transformer 407 has receive (RX) and transmit (TX) nodes forcoupling to fax chip set 201 via interface ports 202. Relays 401-405electrically couple external device ports 221, 222, 223, fax chip setinterface port 202, ring generation circuit 305 and off-hook detectioncircuit 305 in a desired configuration in accordance with Table 1 below.

                  TABLE 1                                                         ______________________________________                                        Relay                                                                              Relay  Relay  Relay                                                                              Relay                                                 401   402    403    404  405    Configuration                                 ______________________________________                                        OFF  OFF    OFF    OFF  OFF  G3 fax to fax Chip Set                           OFF      OFF                                                                                  ON     OFF                                                                                OFF                                                                                STU to PSTN, single line                     OFF      OFF                                                                                  OFF                                                                                 ON                                                                                 OFF                                                                                 G3 fax to PSTN (SFXADP bypass)               OFF      OFF                                                                                  OFF                                                                                 OFF                                                                                 ON                                                                                  Ring signal to G3 fax                       ON      ON     OFF    OFF                                                                                 OFF                                                                                PC/AT to fax Chip Set                        ______________________________________                                    

In operation, relays 401-405 are configured in accordance with a desiredelectrical coupling. In the default operating mode, all relays are OFFand therefore relay circuit 402 couples the TX and RX nodes to port G110 which is adapted to couple to a G3 fax machine. Thus, in the defaultoperating mode, the RX and TX signals of fax chip set 201 are coupled toG3 fax 101.

In a second configuration, relay 403 is ON and relays 401, 402, 404, and405 are OFF thereby coupling port S 112 (which is adapted to couple toSTU 102) to port L (which is adapted to couple to PSTN line 105). Thisconfiguration is selected when there is only a single PSTN lineavailable in order to transmit and receive data from the single PSTN.

In a third configuration, relay 404 is ON and relays 401-403 and 405 areOFF. In this configuration, Port G 110 (adapted to couple to G3 fax 101)is coupled to Port L 113 (adapted to couple to PSTN line 105). Inoperation, this third configuration is selected when bypass of SFXADP100 is desired.

In a fourth configuration, relay 405 is ON and relays 401-404 are OFF.This fourth configuration is selected when SFXADP 100 is in "wake-up"mode. Relay 405 is switched "ON" to isolate G3 fax 101 from RX and TXlines and to instead couple ring generation circuit 406 to G3 fax 101via port G 110.

In a fifth configuration, relays 403-405 are OFF and relays 401 and 402are ON. This fifth configuration couples port H 111 (which is adapted tocouple to personal computer 103) to fax chip set 201.

Referring again to FIG. 3B, off-hook detection circuit 301 has sensingnodes 326, 327, symmetrical current limiting circuits 340, 341 anddetection circuit 342. Sensing nodes PV 326 and NV 327 are coupled to aload such as G3 fax 101 via port G 110. Current limiting circuit 340includes transistor 310, resistor 315 and zener diode 314. Currentlimiting circuit 341 includes transistor 313, resistor 322 and zenerdiode 328. Transistors 310, 313 are current limiting regulators forsupplying positive and negative current respectively. Zener diodes 314,328 are coupled to each current supplying transistor 310, 314 to limitthe current supplied to 16 mA. By limiting the current supplied byoff-hook detection circuit 301, external equipment and devices aresafeguarded against damage. Off-hook detection circuit 301, limitscurrent by creating a high impedance between input voltage nodes 324,325 (±12 volts) and any load that is coupled to sensing nodes 326, 327.

Off-hook detection circuit 301 also includes a detection circuit 342having resistors 316-318, 320-321, transistors 311-312 and capacitor323. Detection circuit 342 generates a logic signal PHNOH/329 indicatingeither an "off-hook" or an "on-hook" detection at sensing nodes 326,327.

When G3 fax 101 is "off hook" sensing nodes PV 326 and NV 327 have a 600Ω load and thus are pulled "low" and transistors 310 and 313 are in the"on" state. The current flowing (sourced) to the load however, islimited using zener diodes 314, 328. Zener diodes 314, 328 can sustain amaximum voltage drop of 2.4 volts and thus limits the current flowingthrough transistors 310, 313 to sensing nodes PV 326 and NV 327. Themaximum voltage drop across resistors 315, 322 equal the maximum dropsustainable by the zener diodes 314, 328 respectively less thebase-emitter drop across transistors 310, 313. The emitter currentsI_(e) of transistors 310, 313 corresponds to the current drawn by the G3fax 101 load. Thus, the load current equals the current flowing throughresistors 315, 322 respectively and is limited by zener diodes 314, 328.When transistors 310, 313 are in the "on" state, transistors 311 and 312are also "on" causing the PHNOH/line to be "low" indicating that theload is "off hook."

When G3 fax 101 is "on hook" sensing nodes PV 326 and NV 327 have a highload impedance and thus nodes PV 326 and NV 327 remain "high." Withsensing nodes PV 326 and NV 327 high, transistors 310, 313 are in the"off" state with negligible collector current I_(c). Because thecollector voltage of transistor 310 is essentially the same as the basevoltage of transistor 311, and because resistor 316 has a greaterresistance than resistor 315, the emitter voltage of transistor 311 isless than the base voltage and thus transistor 311 is in the "off" statewith negligible current flow. Transistor 313 is similarly biased in the"off" state. Because negligible current flows through resistor 317, 320,transistor 312 is biased in the "off" state causing logic signalPHNOH/329 to be "high" indicating that the load is "on hook."

In addition to off-hook detection circuit 301, DAA 200 also has ringdetection circuit 303. Ring detection circuit 303 is coupled relays 402and 403 and detects a ring signal received from either port S 112 (theSTU port) or from port L 113 (the PSTN line). Ring detection circuit 303has decreased power requirements as compared to conventional ringdetection circuits. The power consumption of ring detection circuit 303is reduced using 18 volt zener diodes 330 and 332 and switching diode331. A ring signal greater than 18 volts DC triggers the threshold ofoptoisolator 333, and optoisolator 333 couples a signal of the samefrequency to fax chip set 201. Using zener diodes 330 and 332 andswitching diode 331 to condition the ring signal and optoisolator 333 tocoupled the ring signal to fax chip set 201, the need for a high voltagering signal is eliminated.

Referring now to FIG. 5 there is illustrated a ring generation circuit305 in accordance with the present invention. Ring generation circuit305 generates a pair of square wave ring signals 530, 531 to induce aconnection with G3 fax 101. Because ring detection circuit 303 does notrequire a traditional high voltage ring signal, ring generation circuit305 does not have a bulky and heavy step-up-transformer. Instead, ringgeneration circuit 305 uses transistor double switching circuits 510,511 to generate the pair of square wave ring signals 530, 531 having avoltage swing between +12V and -12V. Ring generation circuit 305generates an absolute voltage of 24V, sufficient to trip the ringdetection circuit 303. Additionally, the frequency of the pair of ringsignals 530, 531 is controllable by microprocessor 220 for adapting tothe various G3 fax requirements of foreign countries.

Ring generation circuit 305 receives input signal FREQ 532 and generatesthe pair of ring signals 530, 531 to drive ring detection circuit 303.In a preferred embodiment, input signal FREQ 532 is a 5 volt square waveat 20 Hz frequency (in conformance with the telephone standard in NorthAmerica) and the pair of ring signals 530, 531 are square waves havingan absolute voltage of 24 volts. Ring generation circuit 305 includestwo complementary switching circuits 510, 511. Switching circuit 510includes a drive circuit 512 and a non-inverting circuit 513. Drivecircuit 512 is a push-pull follower amplifier and includes NPNtransistor 521 serially coupled to PNP transistor 522. The baseelectrodes of transistors 521, 522 are coupled together and to theemitter electrode of transistor 523. Similarly, drive circuit 514 ispush-pull follower amplifier and includes NPN transistor 525 seriallycoupled to PNP transistor 526. The base electrodes of transistors 525,526 are coupled together and to the emitter electrode of transistor 527.Drive circuits 512, 514 are coupled to +12V and -12V voltage supplies.Drive circuits 512, 514 generate the pair of ring signals 530, 531having square waves with opposite voltage swings.

In operation, non-inverting circuit 513 conditions the receivedfrequency signal FREQ 532 using transistors 524, 523. The output ofnon-inverting circuit 513 is coupled to drive circuit 512. When FREQ 532is a "1" or "high" (when the signal has a voltage above the thresholdrequired to turn transistor 524 on), transistor 524 conducts and turnson transistor 523 so that transistor 523 is also conducting. Conversely,when FREQ 532 is "0" or low, transistors 524 and 523 are off and notconducting. Transistor 523 drives transistor pair 521 and 522 togenerate an output which follows FREQ 532.

Switching circuit 511 inverts and amplifies the received input (FREQ532). Inverting circuit 515 couples FREQ 532 to drive circuit 514 usingtransistors 527, 528 and 529 to invert FREQ 532. In operation, if FREQ532 is low, the output of inverting circuit 515 is high and thus theoutput of drive circuit 512 will also be high. If, however, FREQ 532 ishigh, the output of inverting circuit 515 is low and thus the output ofdrive circuit 512 will also be low.

In operation, when a user transmits using G3 fax 101, G3 fax 101switches from the "on-hook" state to the "off-hook" state. Responsive tothis change of state, off-hook detection circuit 301 senses that G3 fax100 is "off-hook" and indicates the off-hook state to the fax chip set201 using logic signal PHNOH/329. Responsive to fax chip set 201receiving an "off-hook" indication on logic signal PHNOH/ 329,controller 220 configures DAA 200 and sends a command to fax chip set201 for data reception and re-transmission.

If, however, G3 fax 101 is in the "on-hook" state and SFXADP 100receives an external transmission, SFXADP 100 must "wake-up" G3 fax 101to place G3 fax 101 in a state to receive incoming data transmission.SFXADP generates a ring signal to wake up G3 fax 101. Upon receiving thering generation "wake-up" signal, G3 fax 101 changes from the "on-hook"state to the "off-hook" state. When off-hook detection circuit 301senses that G3 fax 101 is "off-hook" logic signal PHNOH/329 indicatesdetection of the off-hook state to fax communication circuit 201.Responsive to fax chip set 201 receiving an "off-hook" indication onlogic signal PHNOH/329, controller 220 configures DAA 200 to reformatthe received external transmission and couple the reformatted data to G3fax 101.

What is claimed is:
 1. A Data Access Arrangement (DAA) apparatus for afax adapter having a fax communication circuit, a controller and aplurality of external device ports including a fax port and a telephoneport, the DAA apparatus comprising:an off-hook detection circuit havingan input coupled to the fax communication circuit and having an outputcoupled to the controller; a ring detection circuit having an input andhaving an output coupled to the fax communication circuit; a ringgeneration circuit having an input coupled to the controller and anoutput; and a relay circuit having a plurality of control inputs coupledto the controller, the relay circuit further comprising:a first relayhaving a first port coupled to the fax communication circuit and asecond port coupled to the off-hook detection circuit for selectivelycoupling the fax communication circuit to the off-hook detection circuitresponsive to at least one of the plurality of control inputs; a secondrelay having a first port, a second port and a third port coupled to thering generation circuit, the second relay for selectively coupling thefirst port to the second port and to the ring generation circuitresponsive to at least one of the plurality of control inputs; a thirdrelay having a first port coupled to the fax port, a second port coupledto a first port on the second relay, and a third port, the third relayfor selectively coupling the ring generation circuit to the fax portresponsive to at least one of the plurality of control inputs; a fourthrelay having a first port, a second port coupled to the third port onthe third relay and a third port coupled to the telephone port; and afifth relay having a first port coupled to the fax communicationcircuit, a second port coupled to the second port on the second relayand a third port coupled to the first port on the fourth relay forselectively coupling the fax communication circuit to the telephoneport.
 2. The apparatus of claim 1 wherein the external device portsfurther comprise a PSTN line port coupled to the third port of the fifthrelay.
 3. The apparatus of claim 1 wherein the ring generation circuitcomprises a square wave generation circuit.
 4. The apparatus of claim 1wherein the off-hook detection circuit is current limited.
 5. Theapparatus of claim 4 wherein the current limited off-hook detectioncircuit comprises:a first and second sensing node; a first currentlimiting transistor having collector, emitter and base electrodes, thecollector coupled to the first sensing node, the emitter coupled to afirst voltage terminal by a first resistor and the base coupled to thefirst voltage terminal by a first diode; a second current limitingtransistor having collector, emitter and base electrodes, the collectorcoupled to the second sensing node, the emitter coupled to a secondvoltage terminal by a second resistor and the base coupled to the secondvoltage terminal by a second diode; and a detection circuit having afirst input coupled to the first transistor emitter and an output havinga first logic state indicating an off-hook condition at the sensing nodeand a second logic state indicating an on-hook condition at the sensingnode.
 6. The apparatus of claim 3 wherein the square wave generationcircuit comprises:an input node coupled to the controller for receivinga square wave; a first switching circuit coupled to the input node, thefirst switching circuit having a non-inverting signal conditioningcircuit for conditioning the received square wave and having anamplifier coupled to the non-inverting signal conditioning circuit togenerate an amplified conditioned square wave; and a second switchingcircuit coupled to the input node, the second switching circuit havingan inverting signal conditioning circuit for inverting and conditioningthe received square wave and having an amplifier coupled to theinverting signal conditioning circuit to generate an amplified invertedsquare wave.
 7. A method of coupling transmission data to a fax havingan off-hook state and an on-hook state, comprising:generating a squarewave fax wake-up signal; coupling the square wave fax wake-up signal tothe fax; after coupling the square wave fax wake-up signal to the fax,monitoring the fax to detect the off-hook state; receiving a frequencyinput signal; and controlling the frequency of the square wave faxwake-up signal responsive to the frequency input signal.
 8. A DataAccess Arrangement (DAA) apparatus for a fax adapter having a faxcommunication circuit, a controller and a plurality of external deviceports, the DAA apparatus comprising:a relay circuit having a pluralityof inputs and a plurality of outputs and a control input, the relaycircuit being adapted to selectively couple the plurality of inputs tothe plurality of outputs responsive to the control input; an off-hookdetection circuit coupled to the fax communication circuit and to therelay circuit, the off-hook detection circuit comprising:a first andsecond sensing node; a first current limiting transistor havingcollector, emitter and base electrodes, the collector coupled to thefirst sensing node, the emitter coupled to a first voltage terminal by afirst resistor and the base coupled to the first voltage terminal by afirst diode; a second current limiting transistor having collector,emitter and base electrodes, the collector coupled to the second sensingnode, the emitter coupled to a second voltage terminal by a secondresistor and the base coupled to the second voltage terminal by a seconddiode; and a detection circuit having a first input coupled to the firsttransistor emitter and an output having a first logic state indicatingan off-hook condition at the sensing node and a second logic stateindicating an on-hook condition at the sensing node; and a ringdetection circuit coupled to the fax communication circuit; and a ringgeneration circuit coupled to the relay circuit.
 9. The apparatus ofclaim 8 wherein the ring generation circuit is a square wave generatoradapted to generate a square wave ring signal.
 10. A data AccessArrangement (DAA) apparatus for a fax adapter having a fax communicationcircuit, a controller and a plurality of external device ports, the DAAapparatus comprising:a relay circuit having a plurality of inputs and aplurality of outputs and a control input, the relay circuit beingadapted to selectively couple the plurality of inputs to the pluralityof outputs responsive to the control input; an off-hook detectioncircuit coupled to the fax communication circuit and to the relaycircuit; a ring detection circuit coupled to the fax communicationcircuit; and a ring generation circuit adapted to generated a squarewave ring signal, coupled to the relay circuit, the ring generationcircuit including:an input node coupled to the controller for receivinga square wave; a first switching circuit coupled to the input node, thefirst switching circuit having a non-inverting signal conditioningcircuit for conditioning a square wave received from the controller andhaving an amplifier coupled to the non-inverting signal conditioningcircuit to generate an amplified conditioned square wave; and a secondswitching circuit coupled to the input node, the second switchingcircuit having a inverting signal conditioning circuit for inverting andconditioning the received square wave and having an amplifier coupled tothe inverting signal conditioning circuit to generate an amplifiedinverted square wave.